Structure, symmetry, and stability of two-dimensional crystals

Self-assembly of molecules at the liquid/solid interface via physisorption is critical for determining the properties of films, the nucleation of crystals, and selective adsorption behavior from solution. Such self-assembly often results in periodically ordered monolayers---two-dimensional crystals---which provide a means of patterning surfaces on the low-end of the nanoscale. Understanding of the factors controlling the structure and stability of these assemblies, however, is lacking. Currently, it is difficult to predict what packing motif will prove most stable for a given molecule, and thus likely be adopted at the surface. To address this issue, two-dimensional crystal structures and stabilities were examined both experimentally and using an informatics approach.;The structure and stabilities of the two-dimensional crystals formed at the phenyloctanel graphite interface by sets of chemically or geometrically related molecules were compared using scanning tunneling microscopy and computational modeling. Within a series of 1,3-disubstituted benzenes (including 1,3-dinonadecanoylbenzene, 1,3-dioctadec-1-ynylbenzene, 1,3-dioctadecylbenzene, dioctadecyl dithiolisophthalate), the geometry and functionality was subtly varied resulting in marked changes in the packing behavior. Many different packing symmetries, multiple inequivalent molecules in the asymmetric unit, and pseudopolymorphism were all observed. The competitive adsorption between ortho-, meta-, and para-disubstituted benzene isomers of dioctadecyl and diheptadecyl phthalate was examined in the context of the trends in the thermodynamic properties of three-dimensional crystals. The order of stabilities of two- and three-dimensional crystals was the same, but the magnitudes of the differences in adsorption strengths could only be explained with analysis of the two-dimensional crystal structures. A cocrystal with remarkably large periodicity was observed during the competitive adsorption experiments, prompting exploration of the origins of this behavior.;The compilation and analysis of the Two-Dimensional Structural Database (2DSD) enabled the first statistical examination of the crystallographic and molecular properties of two-dimensional crystals. The principle of close-packing was found to apply to monolayer structure as it does to bulk crystal structures, but with differing implications due to the change in dimensionality. The tendency toward close-packing of monolayers influences the selection among the many possible plane groups, the rates of retention and generation of molecular symmetry elements, and induces a tendency toward chiral two-dimensional crystal formation.